1. Field of the Invention
This invention relates generally to semiconductor liquid junction photocells, and in particular, to such photocells using CuInS.sub.2 as the photosensitive electrode.
2. Description of the Prior Art
Concern over the possible depletion of fossil fuel energy resources has generated intense interest in recent years in the search for and development of alternative energy sources. One contemplated alternative energy source is solar energy which may be utilized as electricity either directly through photovoltaic devices or indirectly through thermal devices. The latter approach has not received as much attention as the former which will, as presently contemplated, use semiconductor devices. These semiconductor devices are presently relatively expensive power sources, as compared to fossil fuel power sources, because the devices generally collect light in proportion to the areas of their photosensitive junctions, which must be large to generate useful photocurrents. The manufacturing cost of such devices depends mainly upon the area of the photosensitive junction and is presently too high to permit successful commercial exploitation in other than specialized applications.
Considerable effort has therefore been expended in attempting to reduce the cost of converting solar energy to electricity with semiconductor devices. One approach is to use polycrystalline thin films rather than single crystals as the photoactive elements. Another approach that has generated much interest and enthusiasm recently is a liquid-semiconductor solar cell in which the active part of the cell is a junction formed at a liquid-semiconductor interface. Properties of this type of solar cell were reviewed by Gerischer in Electroanalytical Chemistry and Interfacial Electrochemistry 50, pages 263-274, (1975). Because the junction forms spontaneously at the semiconductor-liquid interface and relatively costly epitaxy or diffusion procedures are not required to form the junction, semiconductor liquid junction solar cells promise economies in manufacture as compared to cells in which the junction is formed between two solids.
Many semiconductors have been investigated as photoactive electrode materials in semiconductor liquid junction cells. Cell stability has been a recurrent problem and the efficiency of the photocell may decline with operating time for any of several reasons. For example, photoexcitation may produce holes at the surface which chemically react with the electrolyte and produce an elemental layer of one of the semiconductor constituents on the electrode surface. Other processes, such as chemical etching or deposition of electrolyte impurities on the semiconductor surface, may also occur. These processes corrode and/or passivate the semiconductor surface and degrade cell efficiency as manifested by a decrease in photocurrent as cell operating time increases. One approach to this problem involves the use of a polychalcogenide/chalcogenide redox couple which consumes the holes in competition with the corrosion reaction. Although it is difficult if not impossible, to accurately predict all degradation mechanisms for a particular semiconductor and take precautions to avoid the mechanisms, such an approach has been successful with CdSe, CdS and GaAs electrodes.
A bandgap between approximately 1.0 and 1.7 ev will theoretically give the most efficient photovoltaic conversion of solar power into electricity and a cell using such a material and producing a stable photocurrent over an extended time period would be extremely desirable from a commercial point of view. Although some semiconductors, such as GaAs, with bandgaps within the desired range have produced stable photocurrent output for extended time periods, a search for other semiconductors capable of producing stable photocurrents for extended time periods continues because of possible economic advantages they may offer.